1. Field of the Invention
The present invention relates to a process for production of a semiconductor device, particularly to a process for production of a semiconductor device having a capacitor part comprising electrodes made of a platinum group metal (e.g. platinum or iridium), which process comprises steps of removing the contaminants derived from the platinum group metal.
2. Description of the Related Art
Memory cells such as DRAM have become increasingly fine in recent years, but it is difficult to allow them to have a sufficient capacity when a conventional nitride or oxide film is used as the dielectric film, because the film has a dielectric constant of only about 2 to 3. In order to solve this problem, there has come to be used a ferroelectric film having an ionic bond type perovskite structure composed of barium titanate, strontium titanate, barium strontium titanate or the like. Also, logic LSI's mounting a ferroelectric random access memory (FeRAM) thereon have come to be widely used in IC cards or general-purpose microcomputers, and ferroelectric films such as PZT (Pb(ZrTi)O.sub.3) and SBT (SrBi.sub.2 Ta.sub.2 O.sub.9) are in use as the capacitive insulating film of the FeRAM. These ferroelectric films, when in direct contact with the substrate of FeRAM, give rise to oxidation of the substrate and consequent deterioration of FeRAM capacity. Therefore, in using such a ferroelectric film composed of the above material, there has come to be used an upper or lower capacitor electrode composed of a platinum group metal (e.g. platinum or iridium) which is non-reactive to the material of ferroelectric film. FIG. 4(a) to FIG. 4(i) are sectional views showing the steps employed in production of a semiconductor memory device which has, in combination, a MOS transistor and a capacitor part comprising platinum group metal electrodes and a ferroelectric film and which can conduct data storage by charge accumulation. Referring to these figures, an example of use of platinum group metal in production of a semiconductor memory device is described specifically.
First, a MOS type transistor is formed on a silicon substrate 1 according to a known method, as shown in FIG. 4(a). That is, there is formed a field oxide 2 as an element-isolating region, by a known method such as LOCOS (Local Oxidation of Silicon); subsequently, there is formed a silicon oxide film as a gate-insulating film 3 in a thickness of about 50 to 100 nm, by thermal oxidation. Then, a P-doped polysilicon film and a WSi film are formed in this order; after which they are subjected to patterning to form a gate electrode 4. Thereafter, ion implantation is conducted to form an impurity-diffused layer 5, whereby a MOSFET is completed.
Next, as shown in FIG. 4(b), there is formed, by CVD, a boron-containing silicon oxide film (BPSG) as a first inter-layer insulating film 6; then, etching is conducted to form a contact hole; in the hole are formed a Ti film and a W film in this order to form a vertical interconnect 7.
Subsequently, as shown in FIG. 4(c), there is formed a platinum thin film which becomes a lower electrode layer 8 of capacitor part; then, there are formed a ferroelectric film 9 (e.g. PZT) and a platinum thin film which becomes an upper electrode layer 10 of capacitor part, in this order.
Next, as shown in FIG. 4(d), a masking layer 11 consisting of a silicon oxide film is formed in order to protect the above-formed capacitor part at a time when other element part is formed.
After the other element part has been formed, a photoresist pattern 12 is formed on the masking layer 11, as shown in FIG. 4(e). Using this resist pattern as a mask, the masking layer 11, the upper electrode layer 10, the ferroelectric film 9 and the lower electrode layer 8 are dry-etched to form a desired capacitor part 14 and leave thereon the masking layer 11, as shown in FIG. 4(f).
Next, the masking layer 11 is removed, as shown in FIG. 4(g). Then, a second inter-layer insulating film 15 is formed so as to cover the capacitor part 14 and the first inter-layer insulating film 6, as shown in FIG. 4(h); thereafter, there is formed, in the film 15, an opening 16 for connecting an upper wiring (to be formed later) to the upper electrode layer 10, as shown in FIG. 4(i).
Lastly, a metal film is formed so as to fill the opening 16 and further cover the whole surface of the material obtained in the step of FIG. 4(i); the metal film is subjected to patterning to form a desired upper wiring 17; thereon is formed a third inter-layer insulating film 18 (e.g. a silicon nitride film), whereby is formed a semiconductor memory device such as shown in the sectional view of FIG. 3.
In the steps of FIG. 4, in conducting etching to form a capacitor part 14 having a masking layer thereon, there was a fear that the etching residue adhered to the side wall of the capacitor part 14, resulting in the reduced property of the device produced. To remove the etching residue, a cleaning operation has been carried out. For example, JP-A-10-12836 discloses a method for removing the etching residue by using, as a cleaning solution, hydrochloric acid, nitric acid, hydrofluoric acid, a mixture thereof, water of 80.degree. C. or higher temperature, or an organic solvent.
In production of a semiconductor memory device, when there is formed, as mentioned above, for example, a silicon oxide film as a protective mask 11 for formation of other element part, or a second inter-layer insulating film 15 so as to cover a capacitor part 14, platinum atoms or particles 13 are generated from the upper platinum thin film 10 of the capacitor part 14 and deposit on the protective mask 11 or the second inter-layer insulating film 15; when there is formed a contact hole communicating with the upper electrode, there adhere, onto the second inter-layer insulating film 15, the platinum atoms or particles vaporized from the upper platinum thin film, a reaction product thereof with chlorine gas (a dry-etching gas), i.e. platinum chloride, and an oxide thereof; further, they adhere even onto the back surface of a silicon substrate 1. Since the platinum atoms, in particular, easily cause thermal diffusion in the silicon substrate, the platinum atoms adhering onto the back surface of the substrate may move to the transistor element region of semiconductor device; this affects a serious problem on the property of the element. A similar problem arises also when, in forming an insulating film on other silicon substrate for production of a semiconductor device by using the apparatus used for formation of the above-mentioned protective mask or second inter-layer insulating film, the platinum atoms or particles remaining in the apparatus adhere onto the insulating film formed on the other silicon substrate or onto the back surface of the other silicon substrate. It is known that the above remaining of platinum contaminants, even if the amount is as small as about 1.times.10.sup.10 atoms/cm.sup.2, adversely affects the life time or electrical property of the semiconductor device produced.
Such contaminants include (1) a metal ion adsorbed by or bonded with the surface of a semiconductor substrate, the surface of an insulating film (e.g. a silicon oxide film) formed thereon, and the back surface of the substrate, and (2) metal particles adhering thereon.
It is necessary, therefore, to remove such contamination by platinum group metal. However, there has been developed almost no effective methods for removing the contamination.
In practical production of a semiconductor device, there is a case that one cleaning tank is used for a plurality of cleaning steps, depending upon the kind of the target substance to be removed. In such a case, if one cleaning tank used for cleaning of a substrate contaminated with a platinum group metal is used for cleaning of other substrate, secondary contamination may take place. Therefore, the contamination of the former substrate by platinum group metal need be removed beforehand. Use of one apparatus for a contaminated substrate and other substrate may be also conducted in various steps such as, oxide film formation step in addition to the above-mentioned cleaning step.
As the conventional chemical solutions for metal removal, there are known, for example, a mixed solution of hydrochloric acid, hydrogen peroxide and water (HCl--H.sub.2 O.sub.2 --H.sub.2 O: HPM); a mixed solution of sulfuric acid and hydrogen peroxide (H.sub.2 SO.sub.4 --H.sub.2 O.sub.2 : SPM); a mixed solution of nitric acid and hydrochloric acid (aqua regia); and a mixed solution of ammonia water, hydrogen peroxide and water (NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O: APM). These conventional chemical solutions for metal removal, however, are used for ordinary heavy metals and are unable to sufficiently remove contaminants such as platinum and iridium having a very low ionization tendency, and are difficult to reduce the contamination level to less than 1.times.10.sup.10 atoms/cm.sup.2 mentioned above. Even if the contaminants are removed from the substrate surface, the removed contaminants are suspended in the cleaning solution used and, therefore, they readhere onto the substrate when the substrate is pulled up from the cleaning solution; after all, the contaminants are difficult to remove.
Techniques of removing a metal from the surface of silicon wafer or removing contaminants from the surface natural oxide of silicon wafer, by using a mixed solution of hydrochloric acid, hydrofluoric acid and aqueous hydrogen peroxide for cleaning of silicon wafer, are disclosed in, for example, JP-A-3-228327 and JP-A-8-31781. Any of these techniques is for removal of contaminants present on silicon wafer and is applied before various elements of device are formed. A technique of removing contaminants at normal temperature using a mixture of HF:HCl:H.sub.2 O.sub.2 :H.sub.2 =1:10:20:100 is disclosed in JP-A-3-228327; and a technique of removing contaminants using a solution obtained by diluting 17% HCl: 25% HF=1:1 100-fold with water and adding H.sub.2 O.sub.2 thereto is disclosed in JP-A-8-31781. These techniques are effective for removal of ordinary metal contaminants but make no suggestion to contamination by platinum group metals such Pt and Ir.
In JP-A-7-45580 is disclosed a technique of removing metal contaminants such copper adhering onto the surface of wafer, by a series of cleaning steps which comprise first removing a natural oxide present on a silicon wafer with dilute hydrofluoric acid, then treating the resulting wafer with a mixture of hydrofluoric acid, hydrochloric acid, hydrogen peroxide and water, and thereafter cleaning with a mixture of hydrochloric acid, hydrogen peroxide and water.
The platinum group metals such as platinum and iridium are stable to hydrofluoric acid. Therefore, when treatment with hydrofluoric acid is conducted first, the contaminants adhering onto the oxide film surface, etc. are detached and transferred into the treating solution simultaneously with the etching of the oxide film but are present in the solution as a suspending component. Therefore, they readhere onto the substrate when it is taken out of the solution and cannot be removed sufficiently.
In JP-A-6-333898 is disclosed a technique of cleaning the surface of a semiconductor substrate using a cleaning solution containing (1) a strong acid and an oxidizing agent for removing the organic substances and inorganic substances remaining on the surface of the semiconductor substrate, (2) a fluorine-containing compound consisting of fluorine-generating fluorosulfuric acid or sulfuryl difluoride, for removing the residue and particles which remain on the surface of the semiconductor substrate when the surface is subjected to very slight etching, and (3) water. In the Examples of the literature is shown a case of removing the residue adhering after the dry etching of polysilicon film. However, no mention is made on the removal of platinum group metal.
In Japanese Patent Application No. 10-263482, the present inventor proposed, in order to remove the contamination by a platinum group metal, of an insulating film formed on a semiconductor substrate, a technique of removing the platinum group metal using a cleaning solution which is a mixture of (1) a chemical solution for metal removal and (2) a very small amount of hydrofluoric acid. According to the above technique, the hypochlorous acid ion or sulfuric acid ion formed in the cleaning system ionizes the platinum group metal and makes it soluble; thereby, the re-adhesion of the platinum group metal is prevented; thus, the contamination by platinum group metal can be reduced to less than 1.times.10.sup.10 atoms/cm.sup.2.
Since a fairly strongly acidic condition is required in order to achieve sufficient removal of contaminants, the above technique may produce an adverse effect depending upon the kind of the semiconductor device produced. Moreover, with the hypochlorous acid ion or sulfuric acid ion alone, although the contaminants on oxide film can be removed, re-adhesion of the removed contaminants cannot be sufficiently prevented.